1. Field of the Invention
The present invention relates, in general, to short-pulse fiber amplifiers, and more particularly to a fiber amplifier which employs a pulse stretcher and a pulse compressor that have mismatched dispersion characteristics. The operating parameters of the amplifier are selected such that nonlinear phase shifts accumulated in the amplifier compensate for third order dispersions resulting from the mismatched dispersions of the stretcher and compressor and vice versa.
2. Description of the Background Art
It is well-known that nonlinear phase shifts (ΦNL) can lead to distortion of short optical pulses. In chirped-pulse amplification (CPA), a pulse is stretched to reduce the detrimental nonlinear effects that can occur in the gain medium. After amplification, the pulse is dechirped, ideally to the duration of the initial pulse. The stretching is typically accomplished by dispersively broadening the pulse in a segment of fiber or with a diffraction-grating pair. For pulse energies of microjoules or greater, the dechirping is done with gratings, to avoid nonlinear effects in the presence of anomalous group-velocity dispersion (GVD), which are particularly limiting. The magnitude of the dispersion of a grating stretcher can exactly equal that of the gratings used to dechirp the pulse, to all orders. At low energy, the process of stretching and compression can thus be perfect. At higher energy, some nonlinear phase will be accumulated and this will degrade the temporal fidelity of the amplified pulse. For many applications, ΦNL (also referred to as the B-integral) must be less than 1 to avoid unacceptable structure on the amplified pulse.
The total dispersion of a fiber stretcher differs from that of a grating pair, and this mismatch results in uncompensated third-order dispersion (TOD), which will distort and broaden the pulse, at least in linear propagation. At wavelengths where the fiber has normal GVD (such as 1 μm), the TOD of the fiber adds to that of the grating pair. Stretching ratios of thousands are used in CPA systems designed to generate microjoule and millijoule-energy pulses, in which case the effects of TOD would limit the dechirped pulse duration to the picosecond range. It has thus become “conventional wisdom” that fiber stretchers are unacceptable in CPA systems and, as a consequence, grating stretchers have become ubiquitous in these devices.
Apart from the difficulty of compensating the cubic phase, a fiber offers major advantages as the pulse stretcher in a CPA system. Coupling light into a fiber is trivial compared to aligning a grating stretcher. The grating stretcher includes an imaging system that can be misaligned, and when misaligned will produce spatial and temporal aberrations in the stretching. A fiber stretcher cannot be misaligned. The fiber is also less sensitive to drift or fluctuations in wavelength or the pointing of the beam that enters the stretcher. Beam-pointing fluctuations may reduce the coupling into the fiber to below the optimal level, whereas they translate into changes in dispersion with a grating pair. Finally, the spatial properties of the beam can influence the stretching with a grating pair, while the pulse stretched in a fiber cannot have spatial chirp, experiences the same stretching at all points on the beam, and exits the fiber with a perfect transverse mode. With fiber amplifiers, there is naturally strong motivation to employ fiber stretchers—grating stretchers detract substantially from the benefits of fiber.
One possible solution to this problem is the combination of a fiber stretcher with a grism pair for dechirping. However, grisms require a challenging synthesis and to date have not found significant use. Recently, significant attention has been devoted to the development of fiber Bragg gratings (FBGs), including chirped FBGs, for use in CPA systems. A chirped fiber grating designed to compensate higher-order dispersion is conceptually the same as a diffraction-grating stretcher, which is matched to the compressor to all orders, but it offers the practical advantages of fiber discussed above. However, it is an experimental fact that the fiber CPA systems that produce the highest pulse energies to date employ ordinary diffraction gratings, not chirped fiber gratings.
In view of the foregoing, there remains a need for a CPA system that can employ a fiber stretcher with a diffraction-grating compressor, but overcomes the cubic phase or third order dispersion problems associated with such arrangements.